1. Field of the Invention
This invention relates to a method of manufacturing a cold-rolled steel sheet suitable for use in parts such as automotive body and so on requiring a press formability particularly a deep drawability. More particularly, it relates to a proper method of manufacturing cold-rolled steel sheet having a high ductility, a small anisotropy in material, and excellent deep drawability, aging resistance and resistance to secondary brittleness under an advantageous application of continuous annealing process.
2. Related Art Statement
In general, press-formable steel sheets have hitherto been manufactured by a box annealing process using a low carbon (C: 0.02-0.07% by weight; abbreviated as "%" hereinafter) Al-killed steel as a starting material, but recently been manufactured by a continuous annealing process using an extremely low carbon steel with C&lt;0.01% as a starting material in order to obtain more improved press formability and high productivity.
In these extremely low carbon steels, carbonitride-forming elements such as Ti, Nb, V, Zr, Ta and the like are added in order to fix C and N soluted in steel, which deteriorate ductility, drawability and aging resistance of the steel sheet. Heretofore, these elements have frequently been added alone since they are expensive. A comparison between properties of Ti and Nb which are most popularly used is as follows.
Ti-containing steel has such advantages that the recrystallization temperature is low, and the mechanical properties such as total elongation (El), Lankford value r-value) and so on are good even when the steel is subjected to a low temperature coiling at not more than 600.degree. C., as compared with Nb-containing steel.
On the other hand, the Nb-containing steel has such advantages that the anisotropy for r-value is small, and the phosphate treating property as a pretreatment for painting is good, as compared with the Ti-containing steel.
In Japanese Patent Application Publication No. 58-107,414 it is disclosed to simultaneously develop advantages of both Ti and Nb. In this case, the upper limit of Ti amount is restricted to (48/12C(%)+48/14N(%)), which is intended to secure a non-aging property and a deep drawability by preferentially consuming a greater part of Ti as TiN and fixing the solute C with the remaining effective Ti (=total Ti-Ti as TiN) and Nb. As seen from a recent press forming for outer parts of automotive vehicles, a stretch forming is mainly carried out rather than a drawing, and particularly steel sheets having a high ductility are more demanded. In this technique, however, El value is within a level of 46.8-48.1% (corresponding to that of mild steel sheet), which is not yet achieved to the satisfactory level.
It has been found that when an experiment is practically conducted within the effective Ti range in accordance with the above technique, C in steel is not effectively bonded to Ti, resulting in the considerable deterioration of ductility and drawability as well as the degradation of aging property through the remaining solute C.